1. Field of the Invention
The present invention relates to an apparatus for spin coating and a method for spin coating, and more particularly to an apparatus for spin coating and a method for spin coating which are used for forming a resist film, an SOG (spin-on-glass) film, a polyimide film, and the like in manufacturing semiconductor devices.
In manufacturing a semiconductor device, a photoresist film is formed as an etching resistant mask to be used in patterning an insulating film and a conductive film, and an SOG film is formed as an interlayer in a tri-layer resist system, and furthermore a polyimide film is formed as a covering insulating film.
These various films are often formed by spin coating and it is desired for them to be made uniform in thickness. As for a photoresist film used as an etching resistant film and an SOG film as an interlayer in a tri-layer resist system, in particular, it is necessary to make them thin and uniform in thickness in case of forming a fine pattern. Therefore, it is desired to further improve an apparatus for spin coating and a method for spin coating.
2. Description of the Prior Art
FIG. 1A is a schematic diagram showing an apparatus for spin coating of resist films according to the prior art.
In FIG. 1A, reference number 1 is a disk-shaped rotatable table for placing a wafer, and the rotatable table 1 is supported by a rotating shaft 2 which is fixed perpendicularly to the wafer placing face of the rotatable table 1 and is connected with a spinning motor 3, so that the wafer placing face may be turned on the rotating shaft 2 by the spinning motor 3. The rotatable table 1 has an unshown suction port in the wafer placing face for fixing a wafer 11 by suction.
Reference number 4 is a process cup set around the rotatable table 1. In the bottom of this process cup 4 are formed an exhaust port 5 for exhausting extra resist which is scattered when applying the resist to the wafer 11, an air current regulating plate 6 which regulates an air current in the process cup 4 in order to form a resist film with uniform thickness by adjusting evaporation of a solvent contained in the resist, and an exhaust port 7 connected with an unshown exhauster.
Reference number 8 is a discharge nozzle, which is connected with a discharge mechanism part 10 through a pipe 9. By means of them, a certain amount of resist solution is fed to the discharge nozzle 8 and discharged on the center part of the surface of the wafer 11.
A method for forming a resist film on the wafer 11 by means of the spin coating apparatus is described in the following with reference to FIGS. 1A and 1B. FIG. 1B is a timing chart showing a process sequence.
First, the wafer 11 is fixed on the rotatable table 1 by suction, and then the inside of the process cup 4 is exhausted through the exhaust port 7 and an air current in the process cup 4 is regulated with the air current regulator plate 6 as an auxiliary means.
Next, after discharging a resist solution of 2 to 3 cc from the resist discharge nozzle 8 on the center part of the wafer 11, the rotatable table 1 is rotated with the spin motor 3. In this case, the resist solution is spread from the center part to the fringe part of the wafer 11 by the centrifugal force. Then, when a rotating speed is increased, the resist solution is spread wider on the wafer 11 by the greater centrifugal force so that the resist solution layer may become thinner and at the same time a solvent in the resist solution is evaporated to form a resist film. This two steps manner of the rotating speed reduces an ununiformity in thickness of the resist film which may be caused in case of an abrupt acceleration at the time of discharging the resist.
When the rotatable table 1 is stopped after a specified period, a resist film of uniform thickness has been formed on the wafer 11 as shown in FIG. 2A.
By the way, in the above-mentioned coating method, since the resist is coated by rotating the wafer 11a, a resist solution is not only spread over the surface of the wafer 11 to the fringe part of it but also scattered from the surface of the wafer 11 by the centrifugal force. For example, an amount of the scattered resist reaches the level of 90% of the whole amount. This is wasteful since the scattered resist cannot be reused.
Further, since a photoresist film may be formed as evaporating a solvent in the resist solution, a state of the coated resist solution, namely, a thickness of the photoresist film varies with the following conditions,
1. The start-up acceleration of the spin motor 3 immediately after discharging the resist, a rotating speed of it, and a rotating period of it PA1 2. An exhausting speed of the air while spin coating, and PA1 3. Ambient temperature, humidity, and the like around each part.
In case of discharging a little amount of resist solution in order to reduce the cost, both scattering of the resist from the surface of the wafer 11 and evaporation of a solvent in the resist solution come to an end before the resist solution reaches thoroughly from the center part to the fringe part of the wafer 11.
Accordingly, the formed photoresist film is not uniform in thickness and the photoresist film cannot reach a specified value in thickness on the fringe part of the wafer 11. Thus there is a problem that a patterned photoresist film formed by exposing and developing the photoresist film varies in a pattern size.
Further, there is also a problem that in an extreme case it is impossible to form a pattern of the photoresist film in the fringe part of the wafer 11 since an area uncovered with photoresist film 12 exists in the fringe part of the surface of the wafer 11. Moreover, in case of reducing content of base polymer in the resist solution by increasing an amount of solvent in order to keep the resist solution in a fluid state for a long time, the formed photoresist film becomes 0.6 .mu.m in thickness which is about half of a specified thickness and as a result a specified film thickness cannot be obtained.